Trunked, mobile communication systems are known. Such systems typically allocate communication resources upon perception of a need for communication services. Such allocation, in some systems, is under control of a resource controller. In other systems, a communication unit searches for an unused communication resource, and upon identification of such resource, seizes possession of such resource.
Upon receipt of an allocation of a communication resource communication units tune to allocated frequencies and begin transceiving an information signal over an allocated bandwidth. Transmissions between communication units or between a communication unit and a subscriber on a public service telephone network may continue until the end of a conversation or until one of the communication units exceeds the range of his transceiver.
Trunked communication systems in which information signals are encoded using quadrature amplitude modulation (QAM) are also known. Such systems combine characteristics of both phase and amplitude modulation to reduce the bandwidth required to carry a fixed amount of information. In QAM, information is conveyed using changes in both the amplitude of a carrier wave and the relative phase angle of the carrier signal with respect to a reference angle. Because of the multi-dimensional aspect of QAM four or more bits of digital data may be transmitted per QAM signal element.
Multi-carrier QAM is a technique in which an information-bearing signal, such as serial digitized voice, or digital data from a computer for example, is divided up into multiple, separate, frequency division multiplexed QAM signals. Each QAM signal occupies a discrete frequency band (with each of the bands being substantially frequency adjacent to the others) and carries a portion of the information in the complete information-bearing signal.
In order to coherently detect the transmitted data, the receiver must be able to measure and correct phase and amplitude variations induced by the transmission channel. The variations may be induced by multiple path signal propagation and are commonly referred to as fading.
Two types of fading can occur over the transmission channel. The types are differentiated by the ratio of the differential delays between multiple signal paths and the transmitted bit period. If the maximum differential delay between significant signal paths is much less than the bit period, then the fading process is referred to as flat fading. The term, flat fading, applies because the channel appears to vary uniformly across the transmission bandwidth as a function of time (i.e. flat across the signal bandwidth). If the maximum differential delay between significant signal paths is comparable to or greater than the bit period, then the fading process is referred to as frequency selective fading. In the case of frequency selective fading the channel variation is a function of frequency within the frequency bandwidth.
The means by which phase and amplitude variations are measured and corrected is based upon a process of inserting known pilot symbols in the transmitted signal stream at fixed intervals. The process of inserting known symbols at predetermined intervals in the data symbol stream is commonly referred to as a time division multiplex (TDM) pilot reference.
At the receiver, the location of the known pilot symbols is determined by a time synchronization process. The difference between the amplitude and phase of the transmitted and received pilot symbols is the variation induced by the channel. An interpolation filter is used to generate channel variation values for the intervening data symbols. The channel variation values are then used to correct for channel variation, resulting in relativly accurate estimates of the transmitted symbols.
When multi-channel transmission is used, each sub-channel may experience its own unique variation due to frequency selective fading. To allow for such unique variation, and to provide a mechanism for correction of such variation, each sub-channel may contain pilot symbols.
In the context of geographic reuse the limiting factor in the quality of a received signal is co-channel interference. Co-channel interference may be caused by reception of unwanted signals on the same frequency as the desired signal.
Attempts to increase the quality of a transmitted signal often include increasing the power level of a transmitted signal. Increasing the power level of a transmitted signal increases the ratio of signal to noise and, consequently, decreases the effect of co-channel interference. Increasing the power level of a transmitted signal often times does result in a higher quality received signal. Increasing the power level, on the other hand, often also results in an expansion of the geographic area within which the resource may not be re-used by other communication systems.
Other attempts to provide a higher quality received signal include implementing search algorithms to identify a signal path exhibiting the least amount of co-channel interference. Such algorithms in some cases involve measuring the quality of a signal received on different communication resources from different sources to identify the resource, and signal source, providing the highest reliability in terms of received signal quality. Methods used in the prior art to identify high reliability resources include signal strength measurements and measurement of bit error rates.
Shown (FIG. 12) is a graph of a computer simulation of probability of bit error versus E.sub.b /N.sub.o (energy per bit divided by noise in a one Hertz bandwidth). The graph, as is known, gives a substantially accurate representation of error rates under a variety of transmitting environments. Transmitting environments offered include S=0 .mu.s delay spread (flat Rayleigh) to S=10 .mu.s delay spread (very bad hilly).
The information shown in FIG. 12, as is known, is also commonly displayed using an ordinate calculated in terms of C/[I+N] where C is signal, I is interference and N is noise. The two methods of displaying bit error rates, as is known to those in the art, are used substantially interchangeably.
In the context of communications between a communication unit and a transceiver at a base site the identification of the highest reliability communication resource is often a measurement of proximity of the closest base site transceiver. The closest base site often provides the strongest signal. Signal strength or bit error rate measurements may often provide similar results in terms of resource reliability when used to identify the communication resource providing access to the closest communication services provider.
Bit error rate calculations, on the other hand, are time consuming. Where error rates are low a significant time interval must be allotted to accumulating and averaging errors.
Signal strength measurements, though quick and easy to implement, do not directly measure resource reliability. Resource reliability in the most direct manner involves minimal bit error rates. Minimal bit error rates, on the other hand, involve resource resistance to short term interference factors such as frequency selective fading or short term signal strength variations.
While, in the past, signal strength or bit error rate measurements have provided good indication of the reliability of a communication resource such measurements do not give indication of the effects of certain types of interference exemplified by frequency selective fading. Frequency selective fading may affect certain aspects of a received signal, thereby degrading signal quality, without appreciably affecting signal strength.
By way of example the following comparison is offered wherein a communication unit compares the reliability of communication resources between the communication unit and two base sites. The maximum allowable bit error rate is two percent. The first base site has a detected bit error rate of 0.5% and 26 dB E.sub.b /N.sub.o with S=0 (flat Rayleight fading). The second base site has a detected error rate of 1.3% and 35 dB E.sub.b /N.sub.o with S=10 .mu.s (very bad hilly). The results can be seen plotted on FIG. 12. The first site shows a 10 dB link margin between the 0.5% error rate (26 db) and 2% error (16 dB). The second site shows a 16 dB link margin between the 1.3% error rate (35 dB) and 2% error (19 dB).
In the example given the lowest bit error rate does not offer the highest reliability in terms of signal reception. While the first site offers a lower bit error rate, log-normal fluctuations in received power levels may disrupt signal reception in excess of allowable standards. Clearly the second site while offering a higher initial error rate offers the highest reliability signalling channel.
While the example offered provides an indication of channel reliability, use of such an algorithm depends upon provisions within the receiver for measuring delay spread. Measurement of delay spread is well known in the art (see "900-MHz Multipath Propagation Measurements for U.S. Digital Cellular Radiotelephone", IEEE Transactions, Vol. 39, No. 2, May 1990, pages 132-139) and allows a receiver to enter such a graph (FIG. 12) based upon measured delay spread and bit error rate for each measured channel. Such an algorithm, as is known, may be used to gain a measurement of reliability of communication resources between base site transmitters and a mobile communication unit.
Measurement of bit error rates, as has been mentioned, is time consuming. Where bit error rates are low a number of measurements may need to be undertaken to ensure reliable data. A communication units seeking handoff may not have time to measure bit error rates in an environment of a rapidly deteriorating received signal.
Because of the importance of mobile communications a need exists for a method of measuring signal quality (reliability of a communication resource) that can be rapidly calculated from easily measured parameters and which takes into account such affects as frequency selective fading. Such a method should give a indication of instantaneous as well as average reliability of communication resources and be subject to rapid evaluation. Such a method would be useful both from the viewpoint of handoff of communication units between base sites but also in terms of selection of an initial base site to initiate a communication transaction.